JP 2008-157742 A (US 2008/0148842 A1) describes a humidity sensor disposed in an intake passage through which intake air is drawn to an internal combustion engine. The humidity sensor outputs a signal according to a humidity of the intake air flowing through the intake passage.
However, if the flow velocity of the intake air flowing through the intake passage is slow, intake air retains near the humidity sensor. In this case, the responsivity of the humidity sensor is lowered.
It is known that an internal combustion engine has an optimal ignition timing according to the humidity of intake air. In recent years, it is required to further raise the fuel mileage and knocking controllability by immediately responding to change in the humidity of intake air so as to control the ignition timing.
In an exhaust gas recirculation (EGR) equipment having a supercharger, failure is caused by condensed water adhering to a compressor. In recent years, condensed water is restricted from adhering to a compressor in the EGR equipment by immediately responding to change in the humidity of intake air so as to control the flow rate of EGR gas, such that the fuel mileage is increased while NOx generation is restricted.
In addition, in urea selective catalystic reduction (SCR) system or NOx storing and reduction catalystic system for a diesel engine, the amount of NOx can be presumed from the humidity of intake air, such that a NOx sensor upstream of the SCR catalyst or NOx storing and reduction catalyst can be replaced with the humidity sensor.